1. Field of the Invention
The present invention relates to a pressure cooker adapted for use in an electric range such as a microwave oven, and a process for making such pressure cookers, wherein the pressure cooker includes a pressure container in which food is confined and cooked at an elevated temperature under high pressure.
2. Description of the Prior Art
Known pressure cookers are provided with a pressure container in which food is cooked at an elevated temperature under a high internal pressure built up by vapor emitting from the food. The high temperature atmosphere raises the boiling point of the food up to a particular cooking temperature. The advantage of a pressure cooker is that tenderness and flavor are imparted to the food. This type of pressure cooker is adapted for use in an electric range, such as a microwave oven, in which the food is cooked at high temperatures generated by microwave.
A typical example of the known pressure cookers will be described in greater detail by referring to FIG. 10:
A pressure cooker 40 includes a pressure container 41 and a lid 42 airtightly covering the container 41 by means of a rubber sealing 43, wherein the container 41 and the lid 42 are made of thermosetting unsaturated polyester resin. The internal pressure is adjusted by a valve 44 so as to achieve an optimum cooking pressure in the pressure container. A safety valve 45 is provided for vapor to escape if pressure builds up beyond a predetermined value.
A pressure effective to cook food (hereinafter called "working pressure") depends upon the nature of foods; common pressure containers work at a working pressure of about 2 atms, others work at about 1.7 atms, and others at about 1.4 atms. By selecting a working pressure, the pressure cooker is used for cooking particular foods. For example, a working pressure of 1.7 atms is suitable for cooking meat, and that of 1.4 atms is suitable for boiling rice.
Other typical examples of the known pressure cookers is shown in FIGS. 12(a) and (b). The illustrated cooker 56 includes a heating plate 50 which is composed of a top layer 51 and a silicon rubber layer 53 with a ferrite layer 52 as a microwave absorber sandwiched therebetween, a tray 54, and a cover 55.
This type of cooker is popular among users, particularly housewives, because of the feasibility of leaving burnt traces on the cooked food as seen in grilled steak. However, a problem is that a thick food is likely to have an uncooked portion in spite of the apparent burnt-traces on the surface. To solve this problem, there is a proposal for steaming the burnt-traced food. However, the steaming requires a sufficiently airtight sealing of the container, otherwise the steam would escape from the container. Also, inadequate steaming spoils the taste and flavor of the food. The steaming process is only suitable for thin food that is previously prepared to some extent.
However, some foods such as unmilled rice, beans, and small bony fish are not palatably cooked at atmospheric pressures. However, most of the known pressure cookers are not designed to operate on an increased working pressure. This is due to the fact that the unavailability of a sufficiently tough material is a bottleneck for constructing such pressure cookers.
A pressure cooker for an electric range such as a microwave oven must satisfy at least the following requirements:
(1) It enables an even radiation of microwaves;
(2) It can withstand higher temperatures than 200.degree. C. in anticipation of any abnormal situations, such as excessive absorption of microwave and extraordinary rise in temperature due to an erroneous switch-on of the heater for the pressure container;
(3) It must have a high breaking strength at 120.degree. C. under 2 atms; and
(4) It must be safe from discoloring, increased breaking strength, etc., which would be likely to cause a hygiene problem.
The known pressure cookers are made of unsaturated thermosetting resin such as polyester which is inherently resistant to heat, but polyester is fragile at increased internal pressures. Other problems are that they are likely to contaminate the cooking food by discoloration occurring at the presence of seasoning ingredients such as soy sauce, and decreased breaking strength mainly due to hydrolysis (a test revealed that the tensile strength was reduced by 50 wt % at 120.degree. C. for 100 hours). This shows that pressure cookers are not suitable for everyday use.